Method of treating a c4 hydrocarbon stream



July 14, 1959 H. v. HEss ET AL 2,894,998

METHOD oF TREATING A c4 HYDROCARBON STREAM Original Filed Feb. 18, 1955 METHOD GF TREATMG A C4 HYDROCARBON STREAM Continuation of application Serial No. 489,232, February 18, v1955. This application `luly 29, 1957, Serial No. 675,518

Unite 2 Claims. (Cl. 260-680) This invention relates to a method of treating a C4 hydrocarbon stream. More particularly, this invention relates to an integrated combination method for handling a C4 hydrocarbon stream such as may be produced in or recovered from a petroleum refinery or hydrocarbon synthesis plant.

In refinery operations it is usually the practice to segregate hydrocarbon streams which may be produced during the refinery operations according to molecular Weight and/or boiling point range. Accordingly, in a modern integrated petroleum refinery which may contain crude oil distillation units, catalytic cracking units, thermal cracking units and/ or thermal or catalytic reforming units there are available, after suitable fractionation, various hydrocarbon or hydrocarbon-containing streams, such as a normally gaseous hydrocarbon stream containing methane and ethane, a C3 hydrocarbon stream, a C4 hydrocarbon stream, a stream containing hydrocarbons having boiling points in lthe gasoline range and the like. The C4 hydrocarbon stream containing n-butane, isobutane, isobutene and sometimes varying amounts of n-butenes has been utilized as a source material for the production of butadiene. Y

When a refinery C4 hydrocarbon stream is utilized for the manufacture of butadiene it usually must undergo a substantial amount of fractionation and, in some instances, chemical treatment, solvent extraction `and `the like in order to produce a suitable feed for the production of butadiene by catalytic dehydrogenation. For instance, a C4 refinery stream comprising n-butane, isobutane, isobutene `and n-butene is fractionated vand treated for the removal of isobutene, by cold acid washing. The isobutane and a substantial amount of the n-butane are separated and recovered las an effluent in extractive clis-Y tillation operations employing liquid furfural as the selective solvent, for the production of a C4 hydrocarbon stream containing a major amount, in the range 50-90 mol percent and higher, on n-butenes for the manufacture of butadiene by catalytic dehydrogenation of the n-butenes. The above-mentioned fractionation, acid treating and extractive distillation operations require a substantial capital investment, in equipment. As indicated, the above-mentioned operations are carried out principally to supply n-butene feed substantially free of non-straight chain hydrocarbons, such as isobutane, and containing only relatively minor amounts of isobutene, in the range -l5 mol percent, for the production of butadiene by catalytic dehydrogenation of the n-butenes.

It is an object of this invention to provide an improved process for the handling and treating of a C4 hydrocarbon stream utilized in the production of butadiene and/or n-butene.

It is another object of this invention to provide an improved process for the manufacture of butadiene from a C4 hydrocarbon stream containing n-butane, isobutane, isobutene and n-butenes.

Still another object of this invention is to provide an improved integrated process for the treatment of a C4 tes Patent vC ice hydrocarbon stream containing normal C4 hydrocarbons and iso-C4 hydrocarbons wherein the normal C4 hydrocarbons are utilized for the manufacture of butadiene and/ or n-butenes land the iso-C4 fraction is substantially exclusively utilized for the production of an alkylate suitable for use as a motor fuel component.

Still another object of this invention is to provide an improved process for the manufacture of butadiene.

In at least one embodiment of this invention at least one of the foregoing objects will be achieved. How these and other objects of this invention are `achieved will become apparent in the light of the accompanying disclosure and drawing which schematically illustrates in a block llow diagram one embodiment of the practice of this invention.

In accordance with this invention we have provided an improved method for handling a C4 hydrocarbon stream containing straight chain vand non-straight chain C4 hydrocarbons wherein said C4 hydrocarbon stream is contacted with a selective adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons to adsorb straight chain hydrocarbons therefrom, thereby producing a treated effluent having a substantially reduced straight chain C4 hydrocarbon content or substantially free of straight chain C4 hydrocarbons which is subsequently treated in a suitable manner to upgrade the same. The adsorbed straight chain C4 hydrocarbons are desorbed from the selective adsorbent to yield a C4 hydrocarbon stream comprised predominantly of or consisting essentially of straight chain C4 hydrocarbons. The desorbed straight chain hydrocarbons are subsequentlyl Itreated, as by catalytic dehydrogenation, for the manufacture of corresponding unsaturated hydrocarbons, such as the n-butenes and/or butadiene.

Referring now to the drawing which schematically illustrates one embodiment of the practice of this invention, a C4 refinery stream containing n-butane, isobutane, isobutene and n-butenes is contacted with a selective adsorbent to selectively adsorb n-butane and the n-butenes therefrom to the substantial exclusion of the isobutane and the isobutene with the resulting production of a treated effluent comprising predominantly isobutane and isobutene. This treated effluent is subsequently subjected to an lalkylation reaction for the production of high octane, branched chain hydrocarbons (alkylate) which are particularly suitable for use in aviation gasolines and as blending components in motor fuels.

The adsorbed straight chain C4 hydrocarbons, compris ing substantially only n-butane and n-butenes, `are desorbed from the selective adsorbent to produce a straight chain C4 hydrocarbon stream comprising predominantly or substantially only n-butane and n-butenes. This stream is then subjected to catalytic dehydrogenation by contact With a suitable catalyst for the production of additional nebutenes and butadiene. The resulting produced buta diene is recovered from the dehydrogenation reaction eiiiuent as a separate product and the unconverted nbutane and n-butenes together with the newly produced n-butenes are recycled to the catalytic dehydrogenation reaction. If desired, at least a portion of the n-butene content of the feed to the catalytic dehydrogenation reactor or the reaction effluent therefrom may be passed to the aforementioned catalytic alkylation operation to maintain a desired isoalkane/alkene ratio therein.

In view of the foregoing description which broadly outlines a practice of this invention, it is apparent that by treating a C4 refinery stream in accordance with this invention for the production of butadiene a single operation, namely selective adsorptive separation of the straight clhain C4 hydrocarbons from the non-straight chain C4 hydrocarbons,i.e., n-butane and n-bntene from isobutane and isobutene, takes the place of the operations of fractionation, acid treatm'ent and solvent extraction or extractive distillation heretofore employed in the preparation of a suitablefeed for butadiene manufacture when starting with a C4 hydrocarbon refinery stream.

FEED STOCKS The practice of this invention is particularly applicable to the treatment of C4 hydrocarbon refinery streams containing n-butane, isobutane and isobutene. The presence of n-butene in the C4 refinery stream is desirable. A C4 hydrocarbon refinery stream might have the composition set forth in Table No. I.

Suitable C4 hydrocarbon streams may be recovered and/ or composited from refinery straight run distillation units, catalytic cracking units, catalytic reforming units, thermal cracking or thermal reforming units, isomerization units and the like.

Selective adsorptive separation of straight chain C4 hydrocarbons from nain-straight chain C4 hydrocarbons Any solid adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of nonstraight chain hydrocarbons can be employed in the practice of this invention. It is preferred, however, to employ as the selective adsorbent certain natural or synthetic zeolites or alumino-silicates, such as a calcium aluminosilicate, Vwhich exhibit the property of a molecular sieve, that is, material or adsorbents made up of porous matter or crystals wherein the pores are of molecular dimension and are of uniform size. A particularly suitable solid adsorbent for straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons is a calcium alumino-silicate manufactured by Linde Air Products Company and designated Type 5A Molecular Sieve. The crystals of this particular calcium alumino-silicate, apparently actually a sodium calcium alumino-silicate, have a pore size or diameter of about 5 Angstrom units, a pore size sufficient to admit straight chain hydrocarbons, such as n-butane and the n-butenes, to the substantial exclusion of the non-straight chain hydrocarbons, such as isobutane and isobutene. This particular selective adsorbent is available in various sizes and forms, c g., n the form of a lAG" and ls diameter pellets or a inely divided powder.

Other suitable selective adsorbents are known and include the synthetic and natural zeolites which, when dehydrated, may be described as crystalline zeolites having a rigid three dimensional anionic network and having interstitial dimensions suliiciently large to adsorb straight chain hydrocarbons but sufliciently small to exclude the non-straight chain hydrocarbons. The naturally occurring zeolite, chabazite, exhibits such desirable properties. Another suitable naturally occurring zeolite is analcite NaAlSi2O6-l-I2O which, when dehydrated and when all or part of the sodium is replaced by an alkaline earth metal, such as calcium, yields -a material which may be represented by the formula (Ca,Na)Al2Si4O12.2H2O Iand' which, after suitable conditioning, will adsorb straight chain hydrocarbons to the substantial exclusion of nonstraight chain hydrocarbons. Naturally occurring or synthetically prepared phacolite, gmelinite, harmotome and the like or suitable modications of these materials by base exchange are also applicable for the practice of this d invention. Other solid adsorbents which selectively adsorb straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons are known. It is contemplated that selective adsorbents may be obtained and prepared by suitable treatment of the various oxide gels, especially the metal oxide gels of the polyvalent amphoteric metal oxides.

The operation for the adsorptive separation of the straight chain C4 hydrocarbons, n-butane and/or n-butene, from an admixture containing the same together with close boiling branched chain C4 hydrocarbons, isobutane and isobutene, is effected by direct contact between the admixture and the particular selective adl sorbent employed. The admixture being separated or treated for the adsorption separation of the straight chain C4 hydrocarbons therefrom may be in the liquid phase or in the gaseous or vapor phase. Any suitable means for effecting liquid-solid contact or vapor-solid contact may be employed. For example, presumably the simplest operation, a liquid mixture containing straight chain and non-straight chain C4 hydrocarbons is contacted with the solid selective adsorbent to form a slurry. After sufficient time has been allowed for the adsorption of the `straight chain C4 hydrocarbons the remaining liquid phase is separated from the slurry and the solid adsorbent subsequently separately treated for the removal or desorption of the straight chain C4 hydrocarbons therefrom.

The selective adsorbent as employed in the adsorptive separation operation may be in the form of a xed bed, a moving bed, a udized bed and the mixture of straight chain and non-straight chain C4 hydrocarbons undergoing treatment may pass in direct concurrent or countercurrent contact with the selective adsorbent.

The adsorption operation may take place at any temperature in the range of 50-700 F. and higher depending upon whether or not it is desired to maintain the mixture undergoing separation in the liquid or vapor phase and depending to some extent upon the composition of the mixture of C4 hydrocarbons and the amount of straight chain C4 hydrocarbons to be removed. Also any desired adsorption pressure may be employed depending upon whether or not it is desired to carry out liquid or vapor phase adsorption. In a liquid phase adsorption operation it is preferred to maintain the adsorption temperature in the range 50300 F., more or less. In a vapor phase adsorption operation it is preferred to maintain the adsorption temperature in the range -5 00 F., more or less.

After the adsorption of the straight chain hydrocarbons has been carried out to the desired extent, the selective adsorbent is treated to desorb and separately recover the straigth chain hydrocarbons therefrom. Desorption of the straight chain hydrocarbons is conveniently effected by the application of heat and is preferably effected by contacting the adsorbent with a hot stripping medium such as flue gas, nitrogen, methane, hydrogen, carbon dioxide, air or oxygen-containing gas under controlled temperature conditions, steam, preferably superheated steam, or any suitable inert normally gaseous material. Generally any gaseous stripping medium is suitable provided it possesses molecular dimensions sufficiently small to permit the entry of the molecules of the desorbing or stripping medium into the pores of the selective adsorbent and thereby desorb or displace the adsorbed straight chain C4 hydrocarbons. Usually a desorption temperature in the range 200-1100 F., preferably a temperature in the range 30G-800 F., is suiicient to effect substantially complete desorption of the adsorbed straight chain C4 hydrocarbons from the selective adsorbent. Although the desorption temperature may be the same as the adsorption temperature, i.e. isothermal adsorption-desorption operations, it is sometimes desirable to maintain the desorption temperature about 10G-300 F. higher than the adsorption temperature. As a general rule -the-desorbing temperature should be highenough so that the adsorbed straight chain C4 hydrocarbons are lrelatively quickly desorbed without at the same time causing destruction of the selective adsorbent or decomposition or cracking of the adsorbed-desorbed straight chain C4 hydrocarbons.

Liquid water, such as superheated water, i.e. water at a temperature greater than 212 F. and at a superatmospheric pressure, may also be employed as a desorbing medium. \It is advantageous to employ steam or superheated steam as the desorbing or stripping medium since, after the desorption of the vstraight chain hydrocarbons has `been effected by superheated steam, the recovery of the straight chain hydrocarbons is effected by simply condensing the steam. Following steam-desorption it is usually desirable to displace the steam now adsorbed to some extent within the pores of the adsorbent by contacting the adsorbent with a purge gas or stripping medium such as a tlue gas, nitrogen, hydrogen, natural gas and the like, preferably at an elevated temperature, in order to sweep the steam from the pores of the adsorbent. It is mentioned that the desorption operation may be carried out at any suitable pressure, higher, lower or equal tothe adsorption pressure. It is preferred,

. however, to carry out the desorption operation at a pressure substantially lower than the adsorption pressure, especially when an elevated adsorption pressure is employed.

After the adsorbed straight chain C4 hydrocarbons have been Ydesorbed from the adsorbent the resulting desorbedregenerated adsorbent -is -now incondition to be contacted with additional Yfresh -feed admixture of straight chain and nonstraight -chain C4 hydrocarbons to effect the desired separation. -In actual practice it has been observed that the selective adsorbent, such as a calcium alumino-silicate, as exemplified by Linde Type 5A Molecular Sieve, may be employed -substantiallyindenitely in the adsorption-desorption operation without evidencing any substantial change in adsorptive capacity.

In accordance with a ,special feature of this .invention the adsorptive separation of the straight chain or n-C4 hydrocarbons is carriedout at a temperature below about .307 F., the critical temperature of n-butane, in order to employ more .effectively the adsorptive capacity of the selective adsorbent. Furthermore in the practice of this invention it is preferred to carry out lthe desorption of the adsorbed n-C4 hydrocarbons at a temperature above about 307 F., the lowest critical temperature the -n-C4 hydrocarbons, including butene-l, butene-Z and butadiene, whose critical temperatures are 320 F., 311 F. and 325 F., respectively. Accordingly by contacting a n-'C4 hydrocarbon stream containing n-butane and an n-butene, eg., butene-"l and/or Ybutene-Z and/or a butadiene with a selective adsorbent of the type described Yhereinabove it is possible to fractionate such a stream into its various constituent n-C4 hydrocarbons by carrying out the adsorption operation at a temperature above the critical temperature of a particular n-C4 hydrocarbon, which particular hydrocarbon under these conditions would be substantially unadsorbed, or at most, only adsorbed :to a minor extent. As an example it is possible to selectively adsorb butene-l and the remaining n-.C4 hydrocarbons over .n-butane by carrying out the adsorption operation at a temperature above the critical temperature of n-butane (307 F.) but below the critical `temperature of the other n-C4 hydrocarbon or hydrocarbons, \e.g. butene-l, 320 F., or butadiene, 325 F. to be adsorbed. With respect to the selective desorption of adsorbed n-C4 hydrocarbons from the selective adsorbent, e.g. vto selectively desorb n-butane over butene-l, both adsorbed in an adsorbent, the desorption 4is carried out at a temperature above 307 F. but below 320 F.

6 vAlkylalioftof the separated non-straight chain C4 hydrocarbons The resulting treated or separated branch chain C4 hydrocarbon effluent comprising predominantly'isobutane and isobutene is subjected to catalytic alkylation for the production of an alkylate which vis particularly suitable as a high octane blending agent in aviation and/ or motor gasoline/s. This treated'eiuent from the Aadsorption operation is particularly suitable as a feed stock to an alkylation reaction since it contains -a substantial amount of isobutane and its corresponding branch chain oleinic hydrocarbon, isobutene. It is realized that depending upon the completeness or thoroughness of the adsorption operation the treated branch chain C4 hydrocarbon efuent may contain minor amounts of straight chain C4 hydrocarbons, such as n-butane and the n-butenes. The presence of n-butenes in the alkylation reaction, however, is desirable. The n-butane would pass through the alkylation reaction substantially unchanged, `acting primarily as a diluent and may be separately recovered from the resulting alkylate. Any suitable catalytic alkylation operation may be employed, such as H2804 catalytic valkylation or HF catalytic alkylation.

Sulfuric acid catalytic alkylation is carried out in the `liquid phase at a relatively low temperature, such as .a temperature in the range .i5-60 F. The liquefied branched chain C4 hydrocarbons comprising isobutane and isobutene, namely a branched chain alkane and a mono-olefin, are contactedwith concentrated, about 98%, Ysulfuric acid whereby isobutane is lalkylated by the isohutene or any other mono-olefin which may be present in the alkylation reaction mixture. Since the alkylation reaction is exothermic the reaction temperature is controlled .and the heat -of reaction is removed by Vautorefrigeration wherein continuous vaporization of the alkylation reaction mixture is permitted. The resulting vaporized gaseous hydrocarbon fraction is compressed, condensed and returned to the alkylation reactor. Another method of effecting temperature control during the alkylation reaction is by means of the so-called effluent refrigeration system wherein the alkylation reaction mix- -ture comprising hydrocarbons and acid is permitted to lsettle to form a hydrocarbon phase and an acid phase. A portion yof the yhydrocarbon phase is separated and ashed at reduced pressure, thereby reducing the temperature of the separated hydrocarbon phase. The resulting relatively cool hydrocarbon phase is then employed as a cooling medium for the alkylation reaction. Following the alkylation reaction the resulting alkylate is washed with caustic to remove the acid catalyst and frac- :tionated for the recovery of the resulting alkylate.

HF alkylation, which is suitable for 'feed stream comprising predominantly isobutane and isobutene, the liqueed reactants are contacted with liquid HF at a suitable low temperature, usually in the range 60-l60 F., to effect `allnylation The alkylation reaction effluent cons1st1r'rg of unreacted hydrocarbons, alkylate and dissolved HF 1s fractionated for the separation of these components and the ultimate recovery of the product alkylate.

During the alkylation reaction, particularly when H2504 is employed as the alliylation catalyst, it is desirable to maintain an isoalkane/alkene or mono-olefin mol ratio 1n the range 1.7-1.l in the fresh feed to the alkylation reaction. If required to maintain this desired ratio additional isobutane and k.isobutene or n-butenes, propylene and C5 and .C6 mono-olens may be added. Desirably additional needed n-butene may be obtained from 'the desorbate from 'the adsorption operation and/ or from the eluent from the dehydrogenation reaction.

Dehydrogenaon of theseparated straight chain C4 hydrocarbons The .straight Vchain C4 hydrocarbons comprising nbutane, .and advantageously n-butenes, after desorption from the selective adsorbent are subjected to catalytic dcn7 hydrogenation for the production of butadiene and/ or nbutenes. When the straight chain C4 hydrocarbon stream kbeing treated comprises a substantial or major amount of or comprises substantially only n-butane it may be catalytically dehydrogenated in a one-step operation to butadiene or, if desired, iirst to n-butene and then subsequently to butadiene. When, however, the straight chain C4 hydrocarbon stream comprises armajor amount of n-butenes it is preferably directly dehydrogenated to butadiene.

Various catalytic dehydrogenation processes are known and are suitable for the catalytic dehydrogenation of nbutane and n-butenes to n-butenes and/ or butadiene. One suitable method is the Houdry butane dehydrogenation process, see The Petroleum Engineer, November 1954, pages C-7 through C-10, the disclosures of Which are hereby incorporated and made part of this disclosure, wherein n-butane is catalytically dehydrogenated to nbutene and/or 'butadiene by vapor phase contact with an active chromic oxide-alumina catalyst consisting of active alumina impregnated with l8-20% by weight chromic oxide. For butadiene production the dehydrogenation reaction is carried out at a temperature of about 1125 F., at a pressure of mm. Hg. absolute and ata throughput of about 1-l.5 vol. feed/vol. cat/hr. For the production of n-butene from n-butane the dehydrogenation reaction is carried out at a temperaturevof about 1125 F., at a pressure of 10 p.s.i.g. and at a throughput of 1.5-2.0 vol. feed/vol. cat/hr.

Another method suitable for the catalytic dehydrogenation of straight chain C4 hydrocarbons for the manufacture of butadiene is particularly applicable to a C4 hydrocarbon stream comprising a major amount of nbutene. In accordance with this method a C4 hydrocar- -bon stream containing a major amount of nebutene (butene-l and/ or butene-2) is contacted in the presence of Superheated steam at an elevated temperature in the range 1050-1300 F. with an iron oXide catalyst whereby at least a portion `of the n-bntenes are converted to butadiene.

In both the above-described catalytic dehydrogenation operations the resulting produced butadiene is recovered by extractive distillation from the 'dehydrogenation reaction euent by contact with a suitable adsorbing medium, such as liquid furfural or cuprous ammonium acetate. Other suitable selective liquid adsorbents may tbe employed, e.g., aniline. The remaining dehydrogenation reaction euent comprising n-butane and/ or nbutenes is returned into the dehydrogenation reaction :for the eventual conversion into butadiene. If the dehydrogenation reaction is carried out substantially only for the production of n-butenes from n-butane liquid furfural selective solvent is employed for the recovery of the nbutenes from n-butane, the n-butane being recycled to the dehydrogenation reaction and the separated n-butenes recovered as a product or passed to a second stage catalytic dehydrogenation reaction for the production of butadiene therefrom.

The following is exemplary of the practice of this invention.

EXAMPLE 1 A fresh feed C4 hydrocarbon refinery stream having is combined with recycled catalytic dehydrogenator reactor 'eiuent comprising about 49 vol. percent butene-l, 17 vol. percent butene-2, 22 vol. percent 11i-butane, 2 vol. percent butadiene and 8 vol. percent iso-C4 hydrocarbons and the 'resulting combined stream is contacted with a solid selective adsorbent to effect substantially complete removal ofthe straight chain C4 hydrocarbons from the non-straight chain C4 hydrocarbons. The resulting treated efuent, now substantially free of straight chain C4 hydrocarbons, has the following approximate composition in vol. percent: isobutane 67% and isobutene 33%. This admixture of isobutane and isobutene is subjected to liquid phase alkylation reaction employing H2SO4 or HF as the alkylation catalyst for the production of aV relatively high molecular Weight, high octane branched chain'alkylate. 'I'he adsorbed straight chain C4 hydrocarbons are desorbed, recovered and catalytically dehydrogenated for the production of butadiene.

Preferably, when treating a combined C4 hydrocarbon stream, as indicated above, or a C4 refinery stream containing less than 50 mol or vol. percent iso-C4 hydrocarbons, such as a C4 stream having a composition substantially the same as that set forth in Table No. II, the C4 stream is prefractionated to produce an overhead fraction containing substantially all the branched chain or iso-C4 hydrocarbons in the C4 stream, Vsuch as an overhead fraction having the composition set forth in Table No. III,

TABLE NO. III

Component: Liquid vol. percent Propane and lighter 5.1 Isobutane 38.8 Isobutene 19.6 Butene-l 18.5 Butadiene 0.3 n-Butane 10.7 Butene-2 7 Pentane and heavier and containing more than 50 vol. or mol percent iso-C4 hydrocarbons, substantially all of the iso-C4 hydrocarbons in the stream so fractionated, and to produce a bottoms fraction containing substantially all of the straight chain C4 hydrocarbons stream so fractionated, such as a bottoms fraction containing more than 50 vol. or mol percent straight chain C4 hydrocarbons, e.g., a bottoms fraction having the composition set forth in Table No. IV.

Y TABLE NO. IV

The overhead fraction is then contacted with the selective adsorbent to remove substantially all ofthe straight chain hydrocarbons therefrom, followed by desorption of the straight chain hydrocarbons and' separation offthe propanerand lighter hydrocarbons therefrom to produce a straight chain C4 stream comprising predominantly nbutane and n-butenes. If desired, this stream may be further treated, as by selective adsorption and/or fractionation or 4by extractive distillation in the presence of liquid furfural, to yield Van eiuentcr raffinate stream comprising substantially only n-butane and an extract streamV comprising lsubstantially only'n-butene.

The lbottoms fraction, after removal of the pentanes and heavier therefrom, may be admixed with the above; described straight chain C4 stream, or theabove-described ranate or extract stream, preferably the extract stream, and passed to the catalytic dehydrogenator. By' operating in the above manner the total amount of hydrocarbons treated by selective adsorption for the separation of isobutane and isobutene is substantially and significantly reduced and is the preferred embodiment of the practice of this invention. The iso-C4 hydrocarbons separated during the above-described operation are sent to the alkylation reactor and the n-C4 hydrocarbons, comprising substantially all of the above-indicated admixture, tgether with minor amount of unseparated iso-C4 hydrocarbons present in the above-indicated bottoms fraction, are sent to the catalytic dehydrogenator.

The resulting admixture comprised predominantly of straight chain C4 hydrocarbons, principally butene-l and butene-2, together with a minor amount of n-butane, preferably less than l0 liquid vol. or mol percent nbutane (based on C4 hydrocarbon content) by separating substantially all of the n-butane from the aforesaid bottoms fraction, is subjected to catalytic dehydrogenation by contact with an iron oxide catalyst (major amount Fe203 promoted with minor amounts of potassia aud chromia) at a temperature in the range 1100-1200 F. for the production of butadiene. A feed admixture of straight chain C4 hydrocarbons to the dehydrogenation reactor might have the following composition in liquid Volume percent: butene-l 31.5%, butadiene 1.1%, n-butane 6.6%, butene-Z 56.6%, iso-C4 hydrocarbons 4.2%. The hydrocarbons issuing from the dehydrogenation reactor might have the following composition in mol percent: propane and lighter 41.0, isobutane 0.3, isobutene 2.0, butene-l 13.9, butadiene 13.1, n-butane 3.9, butenes-2 25.3, pentane and heavier 0.5. After the dehydrogenation reactor effluent has been subjected to suitable fractionation and solvent extraction for the separation and recovery of butadiene therefrom there is recovered a separate butadiene stream containing at least about 97 mol percent butadiene and about 2% butene-l. The remaining n-butane and n-butene in the dehydrogenation reactor eluent are recycled to the dehydrogenation reactor.

This application is a continuation of copending patent application Serial No. 489,232, led February 18, 1955, now abandoned.

As will be apparent to those skilled in the art in the light of the foregoing disclosure many modifications, substitutions and changes are possible without departing from the spirit or scope of this invention.

We claim:

1. A method of treating a C4, hydrocarbon stream consisting essentially of n-butane, isobutane, isobutene and n-butene which comprises contacting said hydrocarbon stream with a selective adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons to adsorb straight chain hydrocarbons therefrom, recovering from the aforesaid adsorption operation a rst stream comprising isobutane and isobutene, desorbing the adsorbed straight chain hydrocarbons from said adsorbent under conditions to effect the selective desorption of n-butane followed by the selective desorption of n-butene, the selective desorption of n-butane being effected at a temperature not greater than the critical temperature of n-butene, subjecting the resulting selectively desorbed n-butene to catalytic dehydrogenation for the resulting production of butadiene and separating butadiene from the eiuent issuing from the aforesaid dehydrogenation reaction.

2. A method of treating a C4 hydrocarbon stream consisting essentially lof n-butane, isobutane, isobutene and n-butene which comprises contacting said hydrocarbon stream with a selective adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons to adsorb straight chain hydrocarbons therefrom, recovering from the aforesaid adsorption operation a rst stream cornprising isobutane and isobutene, desorbing the adsorbed straight chain hydrocarbons from said adsorbent under conditions to eiect the selective desorption of n-butane followed by the selective desorption of n-butene, the se` lective desorption of n-butane being effected at a temperature not greater than the critical temperature of nbutene, subjecting the resulting selectively desorbed n- =butane to catalytic dehydrogenation for the resulting production of butadiene and separating butadiene from the eflluent issuing from the 'aforesaid dehydrogenation reaction.

References Cited in the le of this patent UNITED STATES PATENTS 2,306,610 Barrer Dec. 29, 1942 2,396,854 Jones Mar. 19, 1946 2,430,972 Black et al. Nov. 18, 1947 2,554,054 Owen May 22, 1951 

1. A METHOD OF TREATING A C4 HYDROCARBON STREAM CONSISTING ESSENTIALLY OF N-BUTANE, ISOBUNTANE, ISOBUTENE AND N-BUTENE WHICH COMPRISES CONTACTING SAID HYDROCARBON STREAM WITH A SELECTIVE ADSORBENT WHICH SELECTIVELY ADSORBS STRAIGHT CHAIN HYDROCARBON TO THE SUBSTANTIAL EXCLUSION OF NON-STRAIGHT CHAIN HYDROCARBON TO ADSORB STRAIGHT CHAIN HYDROCARBON THEREFROM, RECOVERING FROM THE AFORESAID ADSORPTION OPERATION A FIRST STREAM COMPRISING ISOBUTANE AND ISOBUTENE, DESORBING THE ODSORBED STRAIGHT CHAIN HYDROCARBONS FROM SAID ODSORBENT UNDER CONDITIONS TO EFFECT THE SELECTIVE DESORPTION OF N-BUTANE FOLLOWED BY THE SELECTIVE DESORPTION OF N-BUTENE, THE SELECTIVE DESORPTION OF N-BUTANE BEING EFFECTED AT A TEMPERATURE NOT GREATER THAN THE CRITICAL TEMPERATURE OF N-BUTENE, SUBJECTING THE RESULTING SELECTIVELY DESORBED N-BUTENNE TO CATALYTIC DEHYDROGENATION FOR THE RESULTING PRODUCTION OF BUTADIENE AND SEPARATING BUTADIENE FROM THE EFFLUENT ISSUING FROM THE AFORESAID DEHYDROGENATION REACTION. 